There are a variety of standard ways to encode information in a machine-readable code that is either affixed to or applied to the surface of a tangible object. Perhaps the most widespread form of machine-readable code is the barcode, but there are many others. Other forms of machine-readable identification include magnetic stripe, magnetic ink character recognition (MICR), optical character recognition (OCR), optical mark recognition (OMR), radio frequency identification (RF/ID) etc.
While these forms of machine-readable identification are widely used and effective for many applications, they all have the disadvantage that they must occupy a dedicated portion of the physical object that they reside on. For example, if one wishes to apply a barcode or magnetic stripe to an object, the physical implementation of the code must occupy some portion of the object's surface apart from the other information content on the object. For some applications, this limitation does not pose a problem. For many applications, however, the need to locate the code on a dedicated portion of the object is a significant drawback. One drawback is that it requires the user to position the object so that the portion carrying the code can be read. Another drawback is that the code is not aesthetically pleasing, and may detract from the overall appearance of the object. In addition, the placement of the code may require an expensive and cumbersome manufacturing and application process.
Another characteristic of these forms of machine-readable identification is that they are perceptible to the users of the object. Again, for many applications, this characteristic is not a concern, and may in fact be a benefit. In some cases, however, it is a disadvantage for the code to be visually perceptible. As noted above, one drawback is that it detracts from the aesthetic appearance of the object. Another drawback is that it may be more likely to be tampered with.
In some applications, however, it may be advantageous to combine barcodes (or other machine-readable codes) and digital watermarks. Watermarks may be embedded in the information content (e.g., an image, photograph or graphics) or texture of an object's surface, and thus, do not require a separate, dedicated portion of the surface area. While some forms of image watermarks are visible, many others may be embedded such that they are virtually imperceptible to a user, yet readable by a machine.
In the following detailed description, watermarks and related machine-readable coding techniques are used to embed data within the information content on object surfaces. These techniques are used in combination with standard machine-readable coding methods such as bar codes, magnetic stripes, etc. As such, the coding techniques extend to many applications, such as linking documents together, identification card authentication, etc.
Further features and advantages will become even more apparent with reference to the following detailed description and accompanying drawings.